In telecommunication networks, homodyne or direct conversion receivers utilise quadrature mixing for digital modulation schemes (phase and frequency modulation). To support such modulation, the RF (radio frequency) signal or local oscillator (LO) signal is phase shifted by 90° degrees. This results in two signals which are orthogonal in relation to each other, denoted an I signal and a Q signal.
Ideally, the I and Q signals should be of equal amplitude and have a phase shift relative each other of 90 degrees. A deviation in amplitude or phase shift from the ideal can corrupt the down converted signal and raise bit error rate.
The LO chain comprising the local oscillator and circuitry to provide two phase shifted square waves is one potential source of phase mismatches. One reason for this is that the LO chain normally involves in signal handling of high frequencies, often double the frequency of the RF signal. Most of the circuit design of LO chain involves some layout or design that partially differs between I and Q branches. The interface to the external LO signal normally involves impedance transformation or unbalanced to balanced conversion that may introduces error in the LO signal chain.